function [] = circle_theta(radius,theta)
% 绘制绕x轴旋转theta角度得到的圆
% 不能等于PI/2
x=linspace(-radius,radius,50);
y=sqrt((radius^2-x.^2)/(1+tan(theta)^2));
z=y.*tan(theta);
plot3(x,y,z,'k');
hold on
plot3(-x,-y,-z,'k');
hold off
end